Close-boiling light hydrocarbon mixtures with positive deviations from ideal solution behavior, such as ethylene/ethane, propylene/propane, and 1-butylene/n-butane, are normally distilled to highly purify the more volatile olefin component to satisfy downstream processing requirements. See Barclay et al., "Relative Volatilities of the Ethane-Ethylene System from Total Pressure Measurements," Journal of Chemical Engineering Data, Vol. 27, No. 2, pp. 135-142 (1982); Howat III et al., "A New Correlation of Propene-Propane Vapor-Liquid Equilibrium Data and Application of the Correlation to Determine Optimum Fractionator Operating Pressure in the Manufacture of Polymerization-Grade Propene," Industrial Engineering Chemical Process Des Dev., Vol. 19, No. 2, pp. 318-323 (1980); and Sage et al., "System 1-Butene-n-Butane, Composition of Coexisting Phases," Industrial Engineering Chemical Process Des. Dev., Vol. 40, pp. 1299-1301 (1948). The vapor-liquid equilibria for mixtures like these lead to distillation conditions with high reflux ratios where heat pumping and interreboiling can be combined in economically advantageous ways. As an example, an ethylene/ethane separation is normally a step at the end of a low temperature process for the separation of hydrocarbons, e.g. in the recovery of ethylene from the effluent of a pyrolysis cracking furnace.
It is known that ethylene can be separated from ethane by conventional distillation with an externally heated reboiler and an externally cooled condenser. See Kniel et al., Ethylene, Keystone to the Petrochemical Industry, Marcel Dekker, Inc., New York, Chapter 7, at pp. 98-100 (1980). A disadvantage of this process is that, because of the isobaric composition dependence of the relative volatility of ethylene to ethane (see FIG. 3), and because no interreboiler is used, the stripping section operates at an internal reflux ratio much higher than necessary. Another disadvantage is that separate condensing and reboiling heat exchangers must be used to exchange heat individually with the respective external cooling and heating mediums.
It is also known that ethylene can be separated from ethane using conventional distillation with an externally heated interreboiler. See King, Separation Processes, 2nd Edition, McGraw-Hill Book Company, New York, pp. 692-710, at 702 (1980). A disadvantage of this process is that separate condensing, interreboiling, and reboiling heat exchangers must be used to individually exchange heat with the external cooling and heating mediums.
It is also known that ethylene can be separated from ethane using interreboiled distillation with heat pumping where the working fluid is the overhead vapor, an external fluid, or the reboiler liquid. See King, at page 696. This process has different disadvantages depending on the choice of the working fluid. When the working fluid is the overhead vapor, the column pressure must be low enough to avoid approaching critical conditions in the reboiler, thus requiring expensive alloy steel construction of the column and compressor; and, under these conditions, the product ethylene must be cooled to excessively low temperatures. When the working fluid is the overhead vapor, the ethylene product can also become contaminated by process leaks in the heat pump circuit. When the working fluid is an external fluid, separate condensing, interreboiling, and reboiling heat exchangers must be used to individually exchange heat with the external working fluid. When the working fluid is the reboiler liquid, heat absorbed by the interreboiler from the overhead vapor must be transferred indirectly through the working fluid.
It is also known that ethylene can be separated from ethane using multieffect distillation. See King, at pages 694 and 697-700. A disadvantage of this process is that, because no interreboilers are used, the stripping sections operate at internal reflux ratios much higher than necessary. Another disadvantage is that two or more tall distillation columns are required. Another disadvantage is that, because of the low pressure and temperature, at least one column and one heat exchanger must be constructed of expensive alloy steel. Another disadvantage is that, because of the large temperature difference from the low pressure condenser to the high pressure reboiler, conventional heat pumping is not practical.
It is also known that ethylene can be separated from ethane using a dual pressure fractionation tower. See Hayward, U.S. Pat. No. 3,783,126. A disadvantage of this process is that, because no interreboiler is used, the stripping section operates at an internal reflux ratio much higher than necessary. Another disadvantage is that, because heat pumping is not used, separate condensing and reboiling heat exchangers are used to exchange heat with the respective external cooling and heating mediums.
It is also known that ethylene can be separated from ethane using secondary reflux and vaporization where the stripping and enriching sections are at different pressures, allowing heat to be directly exchanged between individual plates in the stripping section and individual plates in the enriching section. See King, at pages 707-708. A disadvantage of this process is that, because the relative volatility of ethylene to ethane is significantly greater in the stripping section than in the enriching section, the temperature differences between the individual plates exchanging heat are not uniform, thus requiring a large pressure difference between the two sections with associated large compression costs. Another disadvantage is that, because of the isobaric composition dependence of the relative volatility of ethylene to ethane, intercondensing heat exchange cannot be used effectively in the enriching section.
And, it is also known from King, at pages 708 and 710, that ethylene can be separated from ethane using isothermal distillation where vapors from each stage are compressed and cooled before being fed to the stage above. A disadvantage of this process is that a large number of compression stages are necessary with associated large o capital costs.